It has conventionally been well-known to obtain an impact-resistant polystyrene resin by blending polystyrene, a hard polymer, with a natural or synthetic rubbery polymer or with a graft copolymer of a rubbery polymer as a backbone polymer and a hard resin-forming monomer, such as styrene, graft-polymerized on the rubbery polymer. Blend-type or graft-blend type rubber-modified polystyrene resins obtained by such a conventional process are however still dissatisfactory in impact strength, visual properties of articles to be molded, etc. In addition, graft copolymers obtained by grafting styrene in the presence of a rubbery polymer latex are prone to agglomeration and are hence accompanied by a handling problem.
On the other hand, graft-type rubber-modified impact-resistant polystyrene resins produced by dissolving a rubbery polymer in styrene and then subjecting them to solution polymerization are excellent in impact strength, stiffness, processability, etc. They are hence used as materials for molded articles such as components of electric appliances, packaging containers, toys and games and sundries.
Although the conventional impact-resistant polystyrene resins are superior in impact strength to polystyrene not subjected to rubber modification, their impact strength is inferior to that of acrylonitrile-butadiene-styrene copolymer resins (ABS resins). The conventional impact-resistant polystyrene resins are thus accompanied by a drawback that a limitation is imposed on their use in a field where a high degree of strength is required, such as car components and the like. Moreover, rubber-modified polystyrene resins are accompanied by a further drawback that their molded products are lower in gloss and less attractive in visual properties compared with polystyrene and ABS resins. It has been proposed to use a styrene-butadiene block copolymer as a rubbery polymer in order to improve the impact resistance of a rubber-modified polystyrene resin (Japanese Patent Publication Nos. 21012/1977, 30739/1984 and 55533/1986). Such a proposal is however still insufficient for obtaining a high degree of impact resistance.